If primary depressives are less sensitive than normal to contingencies of reinforcement, then different contingencies should elicit less marked differences in behavior from depressives than from normal subjects. Furthermore, if the reduced sensitivity depends upon the depressive mood state rather than upon the predisposition to depression, then the behavior of depressives should be more influenced by contingencies after successful treatment. A battery of tasks will be developed for presentation to subjects under varying contingencies of reinforcement in a carefully controlled laboratory situation. Depressed patients will be recruited from the outpatient department. Depressed symptomatic volunteers will be recruited through newspaper advertisements followed by telephone screening interviews. Normal volunteers to match the depressed patients and volunteers will be recruited through advertisements and screening interviews. All subjects will have a detailed clinical assessment, including psychiatric diagnosis and scaled ratings of depression. All subjects will be tested on the battery of tasks on two successive days. Differences in performance under different contingencies will be measured and compared between subject groups. Depressed patients and symptomatic volunteers will be treated under a standard protocol employing imipramine in flexible dosage plus psychotherapy over a period of nine weeks. During this period, subjects will be reevaluated every three weeks with clinical assessments and the battery of tasks. Each subject's differences in performance under different contingencies will be compared before and after treatment. This work may suggest a mechanism by which functional deficits in CNS catecholamines could produce the clinical picture of depression. It may also suggest a procedure for separating out a homogeneous subgroup of depressives responsive to imipramine.